Some little programming puzzle I found. Apparently Facebook asked this sometime. A message containing letters from A-Z is being encoded to numbers using the following mapping: a -> 1, b -> 2, ... etc. How many decodings of some given number. Takes an input of numeral characters.

numeralDecoderPuzzle

import sys

### Some little programming puzzle I found. Apparently Facebook asked this sometime.
### 
### A message containing letters from A-Z is being encoded to numbers using the following mapping: a -> 1, b -> 2, ... etc. How many decodings of some given number.
###
### Takes an input of numeral characters.


input = sys.argv[1]

def howMany( string, l ):
	if not string.isdigit():
		return "error: howMany takes strings with numeral characters"
	elif l == 0:    #base cases
		return 0
	elif l == 1:    #base cases
		if string == "0":
			return 0
		else:
			return 1
	elif l == 2:    #base cases
		if "0" in string:
			return isValid(string)
		else:
			return isValid(string)+1
	else:    
		 return howMany(string[0:-1], l-1)*(string[l-1] != "0")+howMany(string[0:-2], l-2)*isValid(string[-2:l])  #recursive call
	      
def isValid( tail ):
	if len(tail) != 2:
		print "error: isValid takes strings of length 2 with numeral characters"
	elif int(tail) > 26 or tail[0] == "0":
		return 0
	else:
		return 1
	
	
def decode( message ):
	if "00" in message: #any 00 invalidates the string
		return 0
	else:
		return howMany(message, len(message))

print decode(input)